Aerial photography in the search for premium fuels

Press, Volume CXI, Issue 32587, 22 April 1971, Page 22

Aerial photography in the search for premium fuels

(By

D. R. GORDON.

lecturer in civil engineering, University of Canterbury)

It is rather odd to consider that in undertaking an underground search for premium fuels one of the first actions is to send an aeroplane to fly over the area. This is not because the pilot or his crew have any magical ability to see through the ground, or even any training in geology. They are simply there to take large, highquality photographs in a precise predetermined pattern. Generally they use a specially-designed aerial survey camera, with a contact size of 7in or 9in square, looking straight down towards the ground.

This is an expensive undertaking, but it is only a very small part of the long and involved searches now being done for premium fuels. These searches are usually for oil, but a good deal of the technique is also applicable to the search for coal. The search begins with the preparation of a good map. Although the New Zealand map coverage is good by world standards there are still gaps, even at the scale of one inch to the mile. Also, a number of maps in this iseries were produced before and during the Second World War, under difficult conditions and extreme pressure, ‘so their usefulness is limited. IA reliable map is imperative iin oil and coal searches because a vast amount of information must be collected. The clearest way to show all this data is to relate it to a map. Although computers work with figures almost exclusively, the human brain works much more efficiently with diagrammatic and graphical presentation of data, and geological maps are a good example of this. Once a map has been prepared, the geologist, then determines the geological structure of the ground, and he shows this on the map. Basically oil will be found where it has been trapped beneath some impenetrable layer in the earth. The classic example is the upturned dish formation known as an anticline. Modem oil searches have been forced to consider other more complicated formations underground that could also trap oil. In coal searches rather more importance is placed on the type of rocks in an area. But surface inspections can only tell the geologist a ' limited amount about the ' I subsurface structure. Promis- , ing areas will then be subject to gravity surveys, because ' changes m gravity indicate . significant changes ,in the ground beneath. Still further ; information can be gained from seismic surveys, in 1 which small explosions are ! set off at the surface and ! their “echoes” are recorded ' when they bounce back through the earth to the surface from the various layers underground. The final stage : of the oil search is, of course, ! the most expensive of all. This is the drilling of the well. , . . One of the most obvious ‘ uses of aerial photography J is in the making of maps. J Indeed, aerial photographs « from a camera looking < straight down at the ground ■ are a good approximation to ‘ this. One geologist has referred to them as “instant '

» maps.” Such photographs are . never completely correct , maps, because there are 1 more or fewer errors, dependI ing on topography. The tops > of hills appear further away , from the centre of photo- ' graphs than they would in a ■ map, and slopes facing away ! from the centre of the photo- . graph are fore-shortened. . These errors can be quite significant, but the photograph is generally fairly cor- , rect over small areas. Aerial ' photographs are now used ; almost exclusively in the pro- ‘ duction of maps for general , distribution, and also for ; maps required specifically for ' oil searches. Several areas in ! the Highlands of New Guinea ; have recently been mapped j to a much higher standard than the general mapping ' series, to facilitate oil 1 exploration. ; By viewing a pair of photographs, both covering at least partly the same area , (usually 60 per cent overlap) and looking at one photograph with each eye, we can get a three-dimensional view of the ground. Special instruments enable us to measure . this optical image, and make correct scale drawings of it. These scale drawings are our maps, that can contain all the necessary detail, such as roads and houses, but they can also contain contour lines and height measurements. i These techniques have reduced the cost of mapI making considerably, but maps are still expensive to , make. So some geologists use ' just aerial photographs and enlargements. Perhaps more importantly, geologists also use pairs of aerial photographs to give them the three dimensional images of the ground they are surveying. This can be a considerable help in their work, because the surface shape of the ground is often a very good indication of its over-all structure. For instance, after land has been weathered and eroded for a very long time the soft rocks will have been weathered away, while the hard rocks will have remained relatively complete. The structure, for example, of the Canterbury Plains is quite different from that of Banks Peninsula, and this is reflected clearly in the differences in topography, or surface shape. Indeed, the structure within Banks Peninsula itself is at least partially indicated by its topography. But the relationship between structure and topography is not always clear. It shows best in very old countries. In Australia, for example, particularly in the dry interior, almost every change in structure is reflected by a change in topography.

Therefore before he does any great amount of field work a geologist must spend a considerable amount of time studying the aerial photographs of nis survey area. From these he can find evidence of many structural changes within the ground. These cannot always be properly identified from the aerial photographs, but they can be shown on the geological map. The skill of a geologist as an interpreter of photographs is to spot the significance of what he sees in photographs. First he must find the changes in structure, and second he must identify the meaning of them if this is possible. Such changes are usually visible to an observer on the ground, but sometimes it is possible in aerial photographs to detect structural changes that a geologist

on the ground might very well miss. This is mainly because the aerial view enables one to see more of the country at one time than is possible in any other way.

However, it must be remembered that much of the information acquired from aerial photographs is relative; a change here, another change there, and so on. The geologist must still spend a good deal of time in the field, doing traverses over his area to identify positively what all the changes mean, and to be> able to name the rock types that occur. The importance of the aerial photography lies in its ability to save time, especially in field work. This saving in field work time has been estimated as being as high as 75 per cent. What often happens in practice, however, is that the final survey using aerial photography is not less expensive,

or even quicker, but it will be more reliable and more complete than a survey which has been done without aerial photography.

New Zealand is geologically a much younger country than Australia, so less weathering has occurred. This means that it is not nearly so easy to deduce the geological structure from the shape of the ground surface. This is particularly true on the West Coast, for example, where the high rainfall, strong river and stream patterns and the dense forest cover, all help to reduce the value of aerial photography as a guide to the complex and interesting geological structure there. Consequently our coal industry has not needed or been able to use these new techniques to any great extent. In any case, because a comprehensive survey of New Zealand’s coal reserves was

done earlier this century, there is not a great deal to be learnt about the availability of this fuel.